The present invention relates to a pulse width modulation amplifier for a plurality of channels.
A pulse width modulation amplifier is used to modulate a low frequency signal such as an audio signal with a high frequency carrier signal to convert it into a pulse width modulated signal, amplify the pulse width modulated signal and demodulate it by removing the carrier signal before the signal is applied to a load such as loudspeaker. Such an amplifier can amplify a signal very efficiently and thus it is considered very effective for use in automobile mounted audio equipment.
In such an amplifier designed for two channels, the right channel is composed, as shown in FIG. 1, of an audio amplifier 1, a comparator 2, a pulse amplifier (power amplifier) 3, a low-pass filter 4 and the loudspeaker 5. An output of the pulse amplifier 3 is fed back through resistors R.sub.1, R.sub.2 and a capacitor C.sub.1 to the comparator 2. The left channel, is composed similarly of an audio amplifier, a comparator 2', a pulse width amplifier 3', a low-pass filter 4' and a loudspeaker 5'. An output of the pulse amplifier 3' is fed back through resistors R.sub.1 ', R.sub.2 ' and a capacitor C.sub.1 ' to the comparator 2'. A ramp wave carrier signal d produced by a single oscillator 6 is applied to non-inverting terminals of the comparators 2 and 2' which are used to perform the pulse width modulation of the audio signal. The application of the ramp carrier signal d from the common oscillator 6 to both channels is advantageous in view of the elimination thereby of any beat problems and the reduced cost.
It is to be noted that the circuit of FIG. 1 is not being described herein as prior art and this circuit is discussed herein for the purpose of attaining a full understanding of the present invention. The circuit of FIG. 1 is described in more detail in U.S. Pat. application Ser. No. 247,033 filed Mar. 24, 1981.
In this circuit, when input audio signals a and a' are both zero, rectangular pulse outputs b and b' of the pulse amplifiers 3 and 3' are in phase as shown by FIGS. 2a and 2b, respectively, and thus the rise and fall times of the two signals are coincident. On the other hand, when the audio signals a and a' reach a predetermined level, the pulse outputs of the pulse amplifiers 3 and 3' are modualted and the duty cycles thereof varied in accordance with the input audio signals as shown in FIG. 2c. However, if the waveforms of these channels are coincident due to variations of the duty cycles, the comparators 2 and 2' may affect each other and thereby produce inter-channel distortion.